Correction of chromatic aberration in image pickup optical systems, typified by camera lenses, is achieved by combining a high dispersion lens and a low dispersion lens. In recent years, the trend has been to attempt to achieve greater functionality and compactness through the use of optical lenses with higher refractive indexes for both high and low dispersion lenses within the feasible scope of correction of chromatic aberration.
In addition to manipulating the optical characteristics of the glass constituting a lens, it is also possible to achieve greater functionality and compactness in optical systems by rendering the optical functional surfaces of the lens asymmetric. The precision press molding method is suited to the large-scale production of such lenses.
To improve the productivity of the precision press molding method, it is necessary to first achieve a certain degree of moldability to ensure the productivity of glass materials or preforms for precision press molding. That is, a certain viscosity of glass at a temperature at which crystals do not precipitate, or a certain degree of resistance to devitrification at a temperature at which the glass is of moldable viscosity, is required.
On that basis, it is also necessary to enhance the productivity of precision press molding. Expensive mold materials that are resistant to heat and highly rigid, such as SiC and superhard materials, are employed in the precision press molding method. Pressing molds with molding surfaces that have been precision processed into shapes the reverse of the shapes of the optically functional surfaces of optical elements are employed. As a result, the service lifetime of the pressing mold greatly affects production costs and productivity. Thus, effort has been expended on employing glasses with a low glass transition temperature and sag temperature to lower the temperature during press molding and prevent the thermal deterioration of the pressing mold.
Patent Document 1 discloses an optical glass having a low glass transition temperature (Tg), a high refractive index, low dispersion, and good internal glass quality, that is suited to reheat press molding and/or precision press molding. This optical glass is described as having optical constants within the specific ranges indicated in FIG. 1 of Patent Document 1, a low glass transition temperature (Tg), and good internal glass quality that are achieved by the incorporation of prescribed quantities of SiO2, B2O3, La2O3, Gd2O3, Li2O, and F. Thus it is suited to reheat press molding and/or precision press molding.
The optical glass described in Patent Document 2 is a known optical glass having a composition similar to that described in Patent Document 1. An SiO2—B2O3—La2O3—ZnO—ZrO2—F system optical glass with a high refractive index and low dispersion is disclosed in Patent Document 2. This optical glass is described has having a high refractive index, low dispersion, and good devitrification stability. However, there is no description regarding reheat press molding or precision press molding in Patent Document 2. Patent Document 3 describes an SiO2—B2O3—La2O3—Gd2O3—Li2O—F system low transition temperature (Tg) glass preform material having a high refractive index and low dispersion within a prescribed range for use in precision press molding, and an optical glass suited to precision press molding.    [Patent Document 1] Japanese patent publication 2005-170782    [Patent Document 2] Japanese patent publication shou 59-169952    [Patent Document 3] WO2004/015978    Patent Documents 1 to 3 are expressly incorporated herein by reference in their entirety
The differences between Patent Documents 1 and 2 are as follows. The optical glass described in Patent Document 1 contains essentially no As2O3, while all the glasses specifically disclosed in Patent Document 2, with but one exception, contain As2O3. In Patent Document 1, As2O3 is eliminated as a component that greatly burdens the environment.
The SiO2—B2O3—La2O3—ZnO—ZrO2—F system optical glasses described in above-cited Patent Documents 1 and 2 are good optical glasses with high refractive indexes and low dispersion. However, the glasses described in Patent Document 1 comprise a relatively large quantity of expensive Yb2O3, or have a high liquidus temperature, which presents problems in terms of moldability. Many of the glasses disclosed as examples have glass transition temperatures exceeding 600° C., and are not necessarily suitable as optical glasses for precision press molding. The optical glasses of Patent Document 2 have a high glass transition temperature, and cannot be considered suitable for precision press molding. The optical glasses described in above-cited Patent Document 3 present such problems as inadequate thermal stability.
Accordingly, an object of the present invention is to provide an optical glass that solves the above problems and is suited to precision press molding.